1. Field of the Invention
The invention relates to sunscreen compositions including a dibenzoylmethane derivative, such as avobenzone, that are made more stable by the addition of (a) an α-cyano-β,β-diphenylacrylate compound, e.g., octocrylene, and (b) a diester or polyester of naphthalene dicarboxylic acid (e.g., diethylhexyl 2,6-naphthalate), having a weight ratio of (a)/(b) of at least 0.95, preferably at least about 1.0. More particularly, the invention relates to sunscreen compositions which utilize a combination of octocrylene and diethylhexyl 2,6-naphthalate to stabilize other photoactive compounds present in a sunscreen composition and, in particular, to stabilize dibenzoylmethane derivatives, without, or with levels less than 0.5% by weight of, a methoxy-substituted benzophenone derivative, such as benzophenone-3.
2. Brief Description of Related Technology
It is well known that ultraviolet radiation (light) having a wavelength from about 280 nm or 290 nm to about 320 nm (UV-B) is harmful to human skin, causing bums that are detrimental to the development of a good sun tan. UV-A radiation (about 320 nm to about 400 nm), while producing tanning of the skin, also can cause damage, particularly to very lightly-colored or sensitive skin, leading to reduction of skin elasticity and to wrinkles. Therefore, a sunscreen composition for use on human skin preferably includes both a UV-A and a UV-B filter to prevent most of the sunlight within the full range of about 280 nm or 290 nm to about 400 nm from damaging human skin.
Ultraviolet radiation from the sun or artificial sources can also cause harm to coatings containing photoactive substances, such as photoactive pigments and dyes, by breaking down chemical bonds in the structure of a component such as a polymer, a pigment, or a dye. This photodegradation can lead to color fading, loss of gloss, and loss of physical and protective properties of a coating. Photodegradation can take place in several steps which include one or more components of a coating absorbing UV radiation. The absorbed radiation can excite the absorbing molecules and raise them to a higher energy level, which can be very reactive. If the molecule cannot be relaxed, bond cleavage and the formation of free radicals will occur. These free radicals can attack one or more color molecules and/or a polymer backbone and form more free radicals. UV-A and UV-B filters can also be used to accept UV radiation to protect a pigmented coating.
The UV-B filters that are most widely used in the U.S. in commercial sunscreen compositions are paramethoxycinnamic acid esters, such as 2-ethylhexyl paramethoxycinnamate, commonly referred to as octyl methoxycinnamate or PARSOL® MCX, octyl salicylate, and oxybenzone.
The organic UV-A filters most commonly used in commercial sunscreen compositions are the dibenzoylmethane derivatives, particularly 4-(1,1 -dimethylethyl)-4′-methoxydibenzoylmethane (also called avobenzone, sold under the brand name PARSOL® 1789). Other dibenzoylmethane derivatives described as UV-A filters are disclosed in U.S. Pat. Nos. 4,489,057, 4,387,089 and 4,562,067, the disclosures of which are hereby incorporated herein by reference. It is also well known that the above described UV-A filters, particularly the dibenzoylmethane derivatives, can suffer from rapid photochemical degradation, when used alone or when combined with the above-described most commercially used UV-B filters.
The performance of a photoactive compound or a combination of photoactive compounds in a sunscreen composition has been extremely difficult to predict based on the levels of photoactive compounds in the formulation, particularly when the formulation includes one or more photoactive compounds that suffer from relatively rapid photodegradation, such as avobenzone. Because of this, each formulation has required expensive laboratory testing to determine the UV absorbance, as a function of time (quantity) of exposure of the formulation to UV radiation. Moreover, a particularly difficult problem is presented when one photoactive compound in a sunscreen composition acts to increase the rate of photodegradation of another photoactive compound in the composition. This can be accomplished in a number or ways, including a bimolecular reaction between two photoactive compounds and a lowering of the threshold energy need to raise a photoactive compound to its excited state. For example, when avobenzone is combined with octyl methoxycinnamate, a bimolecular pathway leads to the rapid photodegradation of both the dibenzoylmethane derivative and the octyl methoxycinnamate.
Methods and compositions for stabilizing photoactive compounds, such as dibenzoylmethane derivatives, e.g., avobenzone, with diesters and/or polyesters of naphthalene dicarboxylic acid are described in U.S. Pat. Nos. 5,993,789, 6,284,916 and Gers-Barlag, et al. U.S. Pat. No. 6,491,901 ('901). Alternatively, Deflandre, et al., U.S. Pat. No. 5,576,354 and Gonzenbach et al., U.S. Pat. No. 6,033,649 describe the use of an α-cyano-β,β-diphenylacrylate compound, such as octocrylene, to stabilize a sunscreen composition including a dibenzoylmethane derivative. Thus, Deflandre et al. teach a minimum 1% by weight of octocrylene and Gonzenbach et al. teach a minimum of 0.5% by weight of octocrylene to stabilize a dibenzoylmethane derivative, such as avobenzone, in a sunscreen composition. Gers-Barlag, et al. U.S. Pat. No. 6,491,901 ('901) discloses sunscreen compositions containing a dibenzoylmethane derivative with a stabilizing combination of octocrylene and diesters or polyesters of naphthalene dicarboxylic acid wherein the examples have a weight ratio of octocrylene to the diester or polyester of naphthalene dicarboxylic acid in the range of 0.16 to 0.725, and the claims call for a weight ratio between about 0.03 and about 0.9.
In accordance with the '901 patent, the combination of octocrylene with one or more dialkyl naphthalates, according to the '901 invention, acts synergistically to prevent decomposition of avobenzone (see col. 4, lines 21-35). The combination of octocrylene and dialkyl naphthalates of the '901 patent, in the example formulations, disclose 1% to 2.9% octocrylene and 4% to 8% dialkyl naphthalate combinations.
In accordance with the compositions and methods described herein, it has been found that weight ratios of an α-cyano-β,β-diphenylacrylate, e.g., octocrylene, to diakyl naphthalate of at least 0.95, preferably at least about 1.0, provide surprisingly better results for photostabilizing a dibenzoylmethane derivative, such as avobenzone, than lower weight ratios, contrary to the teachings of the '901 patent.
In accordance with another embodiment of the compositions and methods described herein, the formulations contain 0% to about 10%, by weight, to less than 0.05% by weight benzophenone-3, more preferably about 0.1 to about 0.49% by weight benzophenone-3. By including benzophenone-3 in the formulation, a PA+++ rating UVA protection grade, as defined by the Technical Committee of the Japan Cosmetic Association (JCIA) in the Japanese Persistent Pigment Darkening Protocol, hereby incorporated by reference, and attached as an appendix to this application. It should be understood that benzophenone-3 in the formulations represents a separate, preferred embodiment, and is not necessary to provide excellent and surprising photostabilization of one or more dibenzoylmethane derivatives so long as the octocrylene/dialkyl naphthalate weight ratio is at least 0.95, preferably at least about 1.0, as shown in the examples.